I was traveling for work and had to have a rental car for 3-4 months. Every month the rental company called or emailed me requesting the current mileage so that the vehicle could have its maintenance done on it if needed. Assuming the maintenance was done, they were doing their part to stay on top of it.
That being said, my coworker also had a rental car from them. Every time she would shift the car into "Drive" she would push the gear selector sideways so that it would be in third. I had to point out that she shouldn't push the selector sideways, and should instead keep it where it was so that the vehicle will be in normal "Drive". This means that she drove the vehicle for 2-3 weeks, including at least one 1 hour 45 minute drive on the highway (speeds of 70-85 mph), with the vehicle in third. She also wanted to lie about the mileage when they contacted her so that she wouldn't have to take the car back (she didn't want to make the trip to take it back, and she liked the vehicle and didn't want to get stuck with something else).
After seeing her drive, I became more scared of the low standards required to get a driver's license, and I told myself I would never buy a used rental car.